1. Field of the Invention
The present invention relates to a data processing method and an ink jet printing apparatus and more particularly to a data processing method and a printing apparatus that perform a preliminary ejection on a print medium.
2. Description of the Related Art
An ink jet printing apparatus forms an image by ejecting ink from ink nozzles in a print head onto a print medium. Such type of a print head includes one which applies heat to ink by a heater to create a bubble in ink and eject a drop of ink by the pressure of the inflating bubble (Japanese Patent Application Laid Open No. S54-51837) and one that uses a piezoelectric element to eject ink by its mechanical pressure.
In a state in which the ink ejection nozzles are not ejecting ink, water in the ink near an opening of each nozzle readily evaporates, making ink more viscous. Such viscosity increased ink causes ejection failures, such as an ink dot ejection direction being bent or an ink ejection volume becoming too small. Especially, as the nozzle opening diameter decreases, the ejection failure becomes more likely to occur. The ejection failure also occurs due to other causes, such as fine paper powder and water mist adhering to the nozzle opening.
To deal with this problem, it is conventional practice for the ink jet printing apparatus to execute a so-called preliminary ejection, which is not involved in the image forming, immediately before a printing operation or at predetermined intervals during the printing operation. The preliminary ejection discharges viscous ink to forestall possible ejection failures. In a system in which an image is printed on a print medium by scanning a carriage mounting the print head, this preliminary ejection is performed by the carriage moving to a predetermined location, such as a waste ink absorbing member, outside the print medium. Thus, moving the carriage for the preliminary ejection makes the time required for the overall printing operation longer, lowering throughput.
As another preliminary ejection operation, a preliminary ejection operation has been known to eject ink onto the print medium and not to be intended for image forming (hereinafter referred to also as “on-paper preliminary ejection”) (Japanese Patent Application Laid Open No. H06-40042). Particularly, this on-paper preliminary ejection can also be used to ensure that when a nozzle that has not performed the ink ejection for some duration because of print data makes its first ejection after the rest, an ejection characteristic of the first ejection (referred to also as a “first ejection characteristic”) is satisfactory. The on-paper preliminary ejection is normally performed by a single (one drop) ejection and is performed at intervals corresponding to the degree of ink viscosity.
However, in the conventional on-paper preliminary ejection, ejection locations on an image being formed and the number of ejections are preset and thus the preliminary ejection is executed irrespective the position of the image. Consequently, if dots forming an image (image dots) and dots for the one on-paper preliminary ejection (preliminary ejection dots) are printed in vicinity to each other, both dots may be mixed. Because of mixing these dots, a density of the image may increase, degrading a quality of the printed image.